The present invention is related to the field of imaging devices. More specifically, to extending the dynamic range of Time Delay and Integrate (TDI) imagers,
Typical single line imagers, for example, Charge Coupled Device (CCD) imagers, incorporate a single row of photosensitive electrical devices, such as photo-detectors, to collect photonic charge. A photo-detector converts the collected photonic charge into an electrical charge that is related to the amount of photonic charge collected. In moving target imaging the detector integration time is limited by the allowable time the target image moves past the imaging device or scanner. Consequently, as the target image velocity across the imager increases, the allowable time to collect photonic charge decreases. A Time Delay and Integration (TDI) method of operating CCD imagers incorporates multiple single row CCD scanners in an imaging array. Each row in the array collects image data for a finite period of time as the target image and the imager move relative to each other. Typically, the collected charge moves from one row to the next at the same speed as the target image moves across the imager to prevent blurring. TDI-CCD scanners offer an advantage over single line scanners in that TDI-CCD devices provide for longer times to collect and integrate photonic charge from the target image. The longer collection time also increases the sensitivity of the imager.
However, when TDI-CCD devices are used to collect highly intense images, the longer integration time may cause the accumulated charge to exceed the capability of the photosensitive devices. To prevent the excess charge from influencing adjacent photosensitive devices, the excess charge is generally drained away. This draining causes details of an image to be lost. One method of avoiding this loss of detail is to increase the level that is deemed excessive chargexe2x80x94i.e., increase the threshold beyond which charge drainage begins. This method, however, has the disadvantage that the maximum threshold drainage level is set by the saturation level of the photosensitive device. In this case, information is lost not because of draining of the excessive charge, but rather from the inability of the cells to collect any additional charge. Another method to avoid excess charge-accumulation problems is to reduce the amount of time that is used in collecting an image. This method, however, has the disadvantage of reducing sensitivity of the imaging device. Thus, dark images remain relatively dark. Hence, there is a need to extend the dynamic range of TDI-CCD devices that allows for the collection of sufficient image data to view dark images while not losing data from relatively bright images.
In accordance with the principles of the invention, a method is disclosed that increases the intra-scene dynamic range of a TDI-CCD device by selectively limiting the amount of charge collected in each stage by selectively setting blooming barrier levels. By selectively setting barrier levels, the maximum accumulated charge collected in each cell is incrementally increased as the charge is passed from one cell to a subsequent cell. In one embodiment, the level of the barrier is set in a step-wise increasing manner that limits the amount of charge collected in each stage while allowing for the accumulation of progressively greater amounts of collected charge in subsequent cells. In this embodiment, a step-wise increase in the barrier levels changes the reference level at which charge is accumulated in a cell. These changes in the reference level shift the imaging device response characteristic to cause a compression of the accumulated photonic charge for highly intense images while the response to lower intensity images is not substantially affected.